This invention relates generally to materials for lenses, hereinafter referred to as lens materials, and more particularly, to synthetic resin lens materials having high refractive indexes and low dispersion.
Heretofore, a variety of inorganic glass lenses have been used in optical instruments. In addition to such inorganic glass lenses, synthetic resin lenses have begun to be widely used because of their light weight, good processability, high stability, good dyeability, mass productivity, low cost, and other desirable properties.
Among various physical property requirements for a lens, an extremely important requirement is that the refractive index be high and that the optical dispersion be low. A lens having a high refractive index has the following advantages: lens systems which are important components in optical instruments such as microscopes, cameras and telescopes and lenses of spectacles can be made compact and reduced in weight, and aberration at spherical surfaces or the like can be reduced. On the other hand, low dispersion is extremely important in that chromatic aberration can be reduced.
However, in both cases of inorganic glass lenses and synthetic resin lenses, lenses having high refractive indexes tend to have high dispersion which those having low refractive indexes tend to have low dispersion.
For example, the lens material currently most widely used as synthetic resin lenses for spectacles is a diethylene glycol bis(allyl carbonate) resin (hereinafter referred to as CR-39). While CR-39 has an Abbe's number .nu. of 60 (i.e., dispersion is low), the refractive index is extremely low, i.e., n.sub.D.sup.20 =1.50. While polymethyl methacrylate partially used as a lens material also has a high Abbe's number .nu. of 60 as in the case of CR-39, the refractive index is low, i.e., n.sub.D.sup.20 =1.49 -1.50
Polystyrene (n.sub.D.sup.20 =1.59, .nu.=29.0) and polycarbonate (n.sub.D.sup.20 =1.59, .nu.=29.5) which are said to have relatively high refractive indexes and low dispersions are unsatisfactory with respect to other physical properties required for lens materials. For example, polystyrene lacks surface hardness, solvent resistance and the like, whereas polycarbonate lacks surface hardness and impact resistance. While improved polycarbonates (Japanese Patent Laid-Open Pub. No. 89752/1978) and aromatic polyesters (Japanese Patent Laid-Open Pub. No. 110853/1979) have been proposed as synthetic resin lenses having high refractive indexes and low dispersion, the other physical properties described above are not entirely satisfactory. Polynaphthyl methacrylate (n.sub.D.sup.20 =1.64) and polyvinyl naphthalene (n.sub.D.sup.20 =1.68) having high refractive indexes have a low Abbe's number .nu. of 24 and 20, respectively. Thus, all materials are accompanied by various problems.
In recent years, polymers of high refractive indexes containing, as a component, a halogen-substituted aromatic acrylic or methacrylic ester have been proposed (Japanese Patent Laid-Open Pub. No. 7788/1978, Japanese Patent Laid-Open Pub. No. 7789/1978 and Japanese Patent Laid-Open Pub. No. 15118/1980). In these Japanese Patent Laid-Open Publications, however, no mention is made of these copolymers having any Abbe's number, and the halogens used are limited to chlorine and bromine.
In view of the above described state of the art, synthetic resin lenses having a good balance between their refractive indexes and Abbe's numbers have been greatly desired.